The present invention relates to a semiconductor photosensor and a driving method therefor.
The driving method of photosensors is sorted in three types, for example those as illustrated in FIGS. 1(A) to 1(C). In FIG. 1(A), the photosensing system comprises a photosensitive semiconductor device 1 arranged to receive light rays to be sensed, a pulse generator 4 which outputs pulsed signals to the semiconductor device 1, a resistor 2 through which the semiconductor device is earthed, an amplifier 3 inputting the pulsed signal from the semiconductor device 1 and a signal detector 5 receiving the output signal from the amplifier 3. The current level passing through the photosensor is amplified by the amplifier 3 and detected by the detector 5. The output signal levels which would be detected in the case when the semiconductor device is receiving light rays (bright condition) and in the case when the device is not receiving light rays (dark condition) are registered in the detector 5 in advance. The detector 5 carries out the judgement of whether incident light exists or not by comparing the registered level with the actual detected level. FIGS. 1(B) and 1(C) show modifications of the system of FIG. 1(A). Both modifications are based on the differential charge, between dark and bright conditions, accumulated in a capacitor 5 or a integration circuit associated with an I/V convertor 6. These are preferred since charge accumulation is less influenced by noise in comparison with the first type which is based on the current level and unstable due to the hole accumulation effect.
FIG. 2(C) shows the output signal responsive to an input pulse of FIG. 2(B). The solid lines 9 and 12 indicate the output signal in case of bright condition and the dashed lines 10 and 11 indicate the output signal in case of dark condition. However, because of the hole accumulation effect and ensuing reverse current, the light detection performance becomes unstable. Conventionally, some attempts to overcome this shortcoming have been proposed. For example, recombination centers are introduced and built up on the junction of the semiconductor device in order to trap photogenerated holes which tend to linger. Alternatively, the semiconductor region of the device is formed thicker than usual thicknesses so that holes are trapped during moving the elongated drift path to the collector electrode. However, these prior measures provide no basic solution.